This application claims the priority of PCT International Application No. PCT/EP99/00227, filed Jan. 16, 1999 (16.01.99) and German patent document 198 04 988.9, filed Feb. 7, 1998 (07.02.98), the disclosures of which is expressly incorporated by reference herein.
The invention relates to a method for operating a four-stroke internal combustion engine.
Internal combustion engines which operate on the principle of compression ignition of homogeneous lean air/fuel mixtures, offer the possibility of achieving low nitrogen oxide formation and high thermal efficiency. However, these advantages arise only within a narrow operating range which is defined by rapidly changing prevailing conditions.
During compression ignition, the air/fuel mixture is brought to ignition by means of compression heat. After the start of ignition, a self-accelerating combustion process is caused by the energy released initially. Compression which is too low leads to delayed, incomplete combustion, whilst compression which is too high leads to inadmissibly steep pressure rises and to gas oscillations in the combustion space (knocking).
European patent document EP 0 709 562 A2 describes a method for controlling the valve characteristics and the fuel/air mixture of an internal combustion engine. The inlet valves assigned to a cylinder are can be coupled as necessary, so that the fuel/air mixture can optionally be sucked in through one or two inlet valves. The mixture rate can thereby be varied between a lower limit value with a relatively lean mixture and an upper limit value with a relatively rich mixture.
DE-A 195 19 663 discloses a method for operating an internal combustion engine with compression ignition, in which a homogeneous and lean air/fuel mixture produced by external mixture formation is first compressed to near to the ignition limit. In a second step, an additional quantity of the same fuel is finely atomized and is injected into the combustion space, avoiding contact with the wall. The fuel injected late forms a mixture cloud which ignites, because its ignition limit is below the compression temperature reached in the first step, due to the higher fuel content.
EP 0 352 861 A describes a method for operating a four-stroke internal combustion piston engine. The method comprises storing a number of values for determining the various inlet and outlet valve operating times, and the monitoring of a number of existing engine operating characteristics. The value which is most suitable for the existing operating characteristics is selected, and the valve is activated at the time determined by the selected value. In this case, an electronically controlled valve-operated device is activated. The method comprises selectively changing the values on the basis of engine operating experience and, at the same time, using a self-adapting engine control computer. In this case, activation involves a single selective opening and closing of both of the inlet valve and the outlet valve during each complete piston return, only when the engine speed is above a predetermined value. The engine is then operated as a two-stroke engine, using the same inlet and outlet valves as in four-stroke operation.
One object of the invention is to provide a method of the type described previously, which makes it possible for the mixture formation to be adapted quickly to a desired combustion process.
This and other objects and advantages are achieved by the method according to the invention, for operating a four-stroke internal combustion engine with a homogenous, lean basic mixture of air and fuel, utilizing the principle of compression ignition. In the method according to the invention, the fuel/air ratio formed in the combustion space is variable by means of a controllable inlet member. In order to achieve as rapid adaptation as possible to varied combustion sequences, a measurement of the respective combustion is carried out and the time of closing of the inlet member of the combustion space is regulated, for a next successive cycle, as a function of the signal obtained form this measurement.
As a result of the rapid setting of the control times and injection times for each individual combustion space, fluctuations in boundary conditions, such as gas temperatures and component temperatures, are compensated, and optimal combustion is thus implemented for each working cycle in the combustion spaces.
A variable valve control system for the inlet control of the combustion space is provided, by means of which the quantity of the fuel/air mass in the combustion space is varied. Within a cycle of the reciprocating-piston engine, the variable valve control system is activated by control electronics, irrespective of the volumetric state of the combustion space.
The aim of regulating the combustion is to bring about maximum compression. This means that, for the given volume function, in the case of a variable initial quantity of air and fuel, the aim is to achieve maximum filling for the combustion space. Filling is limited by undesirable combustion-space pressure increases which occur, and the subsequent combustion-space pressure oscillations. During a combustion cycle, these undesirable combustion and explosion operations can be detected by assessing the electrical conductivity of the mixture or the solid-borne sound emission of the combustion by means of solid-borne sound sensors (ideally, knocking sensors).
This regulating action is made possible by the use of high-speed adaptive electronics employing stored characteristic maps or neuronal networks. In addition to monitoring data which are indicative of combustion, these electronics also take into account the valve control times and the injection timings, as well as the associated air-excess and residual-gas-fraction values. The influence of the surroundings and of the engine state is detected by means of the intake-air temperature and the cooling-water temperature.